Can I file as self employed?

Tested, if you are "self employed" or an "independent contractor" you may or may not need to pay quarterly estimated income taxes to the IRS, it all depends on your total income for last year, which would include any bank interest, and if you have stock any dividends.   You will know once you completed your income taxes for this year, especially if you need to pay income taxes.

You can file self employed and you should have a form 1099 from him to substantiate what he paid you.